Signature of Planetary Mergers on Stellar Spins

One of the predictions of high-eccentricity planetary migration is that many planets will end up plunging into their host stars. We investigate the consequence of planetary mergers on their stellar hosts' spin period. Energy and angular momentum conservation indicate that planet consumption by a star will spin up the star. We find that our proof-of-concept calculations align with the observed bifurcation in the stellar spin-period in young clusters. For example, after a Sun-like star has eaten a Jupiter-mass planet it will spin up by ∼60% (i.e., spin period is reduced by ∼60%), causing an apparent gap in the stellar spin-period between stars that consumed a planet and those that did not. The spun-up star will later spin down due to magnetic braking, consistent with the disappearance of this bifurcation in clusters (300 Myr). The agreement between the calculations presented here and the observed spin-period color diagram of stars in young clusters provides circumstantial evidence that planetary accretion onto their host stars is a generic feature of planetary-system evolution.